Main drying

Referring to Figure 4.26, three main drying periods ean be identified. 

The cover illustration shows the course of a main drying observed with a cryomicroscope. The photographs are taken after 1.5, 3.0, 4.5, and 6 minutes, respectively. 

If the product temperature approaches T%- from lower temperatures, the viscosity changes rapidly (within a few degrees) by several decimal powers. Since also the product temperature can be measured only with a certain accuracy (see Section 1.2.3), one has to account for both uncertainties. It is recommended that the maximum product temperature during main drying 3 to 5 °C below T. be chosen. 

The idea by Rey to use ER during the main drying for control of the process has not been introduced largely, because as the measurement is not done in a solid block, the product becomes increasingly porous with unknown configurations. The ER measurement of physiological NaCl solution (Fig. 1.24) can be used to check the system. The ER measurement... 

Fig. 1.56.1. Course of main drying observed by a cryomicroscope, in which the freeze drying is carried out. The hydroxy ethyl starch solution is optimally frozen (see Fig. 1.37). The dark lines show the form of the sublimated ice crystals (Fig. 9 from [1.106]).

The author has used a model and an equation developed by Steinbach [1.511 for many years and for many experiments in a wide field of applications. The model, shown in Fig. 1.60, uses an infinitely expanded plate of the product with the thickness d. Equation (14) describes the time of the main drying part of the freeze drying cycle ... 

Fig. 1.60. Schema for the calculation of the drying time fMD for the main drying. The product is frozen in plates (Fig. 6 from [1.51]).

T tot = 7(h - rice, average temperature difference during main drying 43.88 °C... 

Equation (12) describes the main drying reasonable well, if some experimental data are used. 

In this example of the granulate, the main drying time (term A is assumed constant) depends largely on term C, while the vapor transport has virtually no influence. 

The discussions so far about main drying have assumed that trays or vials are exposed to uniform temperatures on the shelves. Kobayshi [1.57] has shown that this condition does... 

Table 1.10.3 Weight loss during main drying of a 5 % mannitol solution in R6- vials.

In Table 1.9 two different plants and three different types of container have been used, in Table 1.10.1 it is always the same plant. During main drying (MD) different pressures have been applied. 

Figure 1.70 shows the three times repeated measurement of desorption rates, without pressure control to demonstrate the reproducibility, and two measurements where the main drying has been pressure-controlled at 0.36 and 0.21 mbar. The process conditions for these five measurements correspond with those in Table 1.10.1. 

The most important data during main drying is the temperature at the moving sublimation front which cannot be measured by Ths or RTDs. In 1958, Neumann and Oetjen 11.651 showed that the barometric temperature measurement (BTM) measures exactly this data. In Fig. 1.77 this is schematically shown if the drying chamber is separated from the condenser by a valve for a short time the pressure in the chamber rises to the saturation vapor pressure (ps) corresponding to the temperature of the sublimation front. ps can be converted into the ice temperature by the water vapor- temperature diagram (e. g. 0.3 mbar = -30 °C). Data for accurate conversion are given in Table 1.11 the temperatures between -100 and -1 °C. 

Table 1. 12 Protocol of Tice during main drying. ...

The main drying in this run could have been terminated after 5 h, but it is useful to wait for the next data, such that the decision is not dependent on one measurement. 

Towards the end of main drying the data of Tice will systematically decrease this effect can be used for an automatic change from main to secondary drying (see Section 2.2.8). 

The condenser design and surface can handle the vapor flow during main drying of this test. The possible low temperatures could be needed during secondary drying. 

Fig. 2.38. DR measurements of the same product with three different operation pressures and therefore with three different Tlce during main drying. The process data were ...

This residual moisture content d W (% of the solids) is calculated as shown in Fig. 2.39. The automatic measurement of DR, which is more accurate than a hand operation of the valve and the visual reading of the pressure gauge, supplies additional information about the main drying as shown in Fig. 2.40.1. The same granulate has been dried at the different pc 0.27, 0.44 and 1.46 mbar corresponding to three Tice -27, -24.5 and -15 °C during main... 

Fig. 2.40.2. Chamber pressure and nitrogen flow rate as a function of drying time. 1 main drying, 2 chamber pressure, 3 nitrogen flow, 4 secondary drying (Fig. 4 from [2.32]).

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